A high degree of uncertainty in characterizing non-CO2 emissions—Several different chamber and field measurement methods could be used to estimate the emissions. Using Intergovernmental Panel on Climate Change factors for ruminant enteric fermentation emissions, for example, is not an accurate method to estimate these emissions. According to Keoleian, many working group participants emphasized the development of new biogeochemical models that help to better characterize emissions.

Heterogeneity in production methods—Different production methods can have different impacts, yet LCA data tend to offer limited resolution of these differences, calling for more extensive research exploiting the heterogeneity.

Non-GHG air pollutant emissions have both regional and local effects—The impacts related to emissions of SO2, NOx, mercury, and other pollutants are more site dependent than GHG emissions, so it is important for the location of the emissions to be inventoried. Unfortunately, many databases do not report emissions in a spatially explicit manner. On the other hand, for carbon emissions and climate change impact, it does not matter where the greenhouse gases are released.

Allocation rules can influence results—Allocation rules are used to distribute impacts from processes with coproducts across the various outputs (e.g., allocating feed production burdens to milk, butter, and hides).

Many participants of the working group recognized that, when considering GHG emissions and energy usage, particularly when considering policies and interventions aimed at reducing GHG emissions or energy usage, one must also consider the human health, environmental health, and economic trade-offs. For example, with respect to the size of a production operation, while some concentrated animal feeding operations (CAFOs)

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